The present invention relates to aqueous film forming foam (AFFF) concentrates that are specially adapted for use on hydrophilic combustible liquids, but are as equally effective for use on hydrophobic liquids. AFFF concentrates are mixtures of surfactants, foam stabilizers and foaming agents which, after diluting with water and mixing with air, form a foam which covers the surface of a burning liquid, thereby enveloping and extinguishing fires on the liquid.
One type of AFFF concentrate is used for fighting hydrophobic fuel fires. AFFF concentrates used for fighting fires on hydrophobic fuels have consisted of a combination of fluorosurfactants, hydrocarbon surfactants and foam stabilizing solvents.
These concentrates have met considerable success in extinguishing fires on hydrophobic liquids, such as hydrocarbons and other non-polar fuels and solvents. AFFF concentrates, once diluted with water and mixed with air, have the ability to spread an aqueous foam on the surface of hydrophobic liquids, thereby extinguishing fires on such liquids.
AFFF concentrates used for fighting fires on hydrophobic liquids are generally diluted with water at a 3-part concentrate to 97-part water ratio. This dilution step is called proportioning. The resulting mixture is then mixed with air and the resulting foam is then applied to the burning hydrophobic liquid. A concentrate which is effective at a 3% dilution level is desired over a weaker concentrate, such as a concentrate which is diluted at a 6 part concentrate to 94 part water ratio, since the user must buy and store more of the weaker concentrate. The use of the stronger concentrate thus reduces storage space and results in reduced costs to the user.
The fighting of fires on hydrophilic liquids, such as alcohols and other polar solvents, is much more difficult than fighting fires on hydrophobic liquids. This is due to the tendency of the foam to dissolve in polar solvents. This problem is diminished by the addition of a water soluble high molecular weight polymer to the concentrate. The water soluble high molecular weight polymer precipitates on contact with the hydrophilic liquid and forms a protective layer, known as a gelatinous mat, which impedes the breakdown of the foam by the polar solvent fuel (see U.S. Pat. Nos. 4,306,979 and 4,060,489). AFFF concentrates containing water soluble high molecular weight polymers are effective on both hydrocarbon and water soluble fuels. Since about the mid 1960's, a polymer typically added to the AFFF concentrate is a high molecular weight polysaccharide, including, but not limited to xanthan gum, guar gum, welan gum and rhamsam gum.
In order to obtain the benefits of the gelatinous mat formed by the high molecular weight polysaccharides it has been found that relatively large quantities of the high molecular weight polysaccharides are needed. While a gelatinous mat will form irrespective of the concentration of the high molecular weight polysaccharides, performance is unsatisfactory at lower concentrations. A drawback to the use of a high concentration of the polymer is that it results in concentrates with a very high viscosity. A typical AFFF concentrate containing a high molecular weight polysaccharide will exhibit a viscosity of 3000-5000 centipoise (cps) when measured using a Brookfield viscometer with a number 4 spindle set at 30 rpm. The high viscosity of the concentrate creates problems in delivery and dilution of the concentrate in foam system applications.